It is known in the tobacco industry to make paperless filter rods using a continuous tape of filtering material, normally cellulose acetate, which is continuously fed through an impregnation station, at which the tape is impregnated with a hardening substance, normally triacetin, and is then transformed, by means of blowing air, into a generally cylindrical tow band, which is caused to advance along a longitudinal through channel of a forming beam comprising a first portion, in this case a stabilization portion, and a second portion, in this case a drying portion. Along the first portion, the hardening substance in the tow band is caused to react by means of blowing steam, normally water steam; while, along the second portion, the tow band, previously moistened by the steam, is dried so as to come out of the forming beam in the form of a continuous rod having a determined stable section and relatively high axial rigidity.
This continuous rod is hence fed, again with continuous motion, to a cutting station to be cut into filter segments of determined length.
The advancing of the tow band along the longitudinal channel of the forming beam is normally obtained by means of a loop conveyor defined by a porous conveyor belt that is permeable to the steam, and comprising a transport stretch extending along the longitudinal channel of the forming beam. The longitudinal channel has a variable section shaped so as to act on the conveyor belt so as to deform it crosswise and cause it to take a tubular configuration wound about the tow band to define, about the tow band, a relatively rigid armature, which on the one hand is permeable to steam and, on the other, tightens about the tow band so as to both give it the determined constant shape of a cross section, and to ensure an axial dragging coupling between tow band and conveyor belt.
Instead, downstream from the forming beam and the mentioned conveyor belt, the newly-formed continuous rod is push advanced, and this type of advancing is only made possible by the fact that, as previously mentioned, the continuous rod is axially rigid.
The need for the continuous rod coming out from the forming beam to be rigid, that is perfectly stabilized and dried, has greatly affected the methodologies used to date for making paperless filter rods and has led to making machines in which, as soon as the mentioned tow band enters the longitudinal channel of the forming beam, it is radially hit by a steam flow supersaturated with a relatively high water flow and content and a relatively low speed; the drying portion is relatively long; and the advancing speed of the loop conveyor is relatively low.
The length of the forming beam and the reduced advancing speed of the loop conveyor allow each section of the tow band to remain in the forming beam for a relatively long time and, in all cases, enough to firstly allow the steam to reach the core of the tow band, due to capillary effect, and cause all the hardening substance to react, and, secondly, the tow band to completely dry as it advances along the drying portion.
Finally, the use of the methodologies known to date has allowed good quality paperless filter rods to be obtained, but with relatively low production speeds.